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Earth Still Stirs Weeks After Tsunami Quake
MELBOURNE (Jan. 9) - Two weeks on, the Earth is still vibrating from the massive undersea earthquake off Indonesia that triggered the tsunami, Australian researchers said on Sunday.
The Australian National University (ANU) said the reverberations were similar in form to the ringing of a bell, though without the sound, and were picked up by gravity monitoring instruments.
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"These are not things that are going to throw you off your chair, but they are things that the kinds of instruments that are in place around the world can now routinely measure," said ANU Earth Sciences researcher Herb McQueen.
"It is certainly above the background level of vibrations that the Earth is normally accustomed to experiencing."
The magnitude 9.0 earthquake, the strongest for 40 years, struck off the coast of Indonesia's Sumatra island on Dec. 26. The tsunami it generated claimed more than 150,000 lives.
McQueen said the oscillation was fading and at current levels equated to about a millimeter of vertical motion of the Earth.
Immediately after the quake the oscillation was probably in the 20 to 30 cm motion range that is typically generated in the Earth by the movements of the sun and moon.
"This particular earthquake because it was 10 times larger than most of the recent large earthquakes is continuing to reverberate," McQueen said.
"We can still see a steady signal of the Earth vibrating as a resultof that earthquake two weeks later. From what it looks like, it appears it will probably continue to oscillate for several more weeks."
The ANU's gravity meter is housed in a fireproof basement at the Mount Stromlo Observatory near the capital Canberra and is part of a global geodynamics project established after major earthquakes in the 1960s.
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U.S. scientists said just after the quake that it may have permanently accelerated the Earth's rotation -- shortening days by a fraction of a second -- and caused the planet to wobble on its axis.
Richard Gross, a geophysicist with NASA's Jet Propulsion Laboratory in California, theorized that a shift of mass towards the Earth's center during the quake caused the planet to spin three millionths of a second faster and tilt about 2.5 centimeters on its axis.
The geologic circumstances that set up one of the worst natural disasters in a century were much longer in the making. How long?
Try 300 million years. Maybe twice that long.
Once, scientists believe, all the Earth's continents were combined in a single gigantic land mass they call Pangea. But geological forces caused it to break apart; and ever since then, the pieces, which scientists call plates, have been drifting across the planet at an average rate of a few inches a century.
As the plates move, grinding collisions between them trigger earthquakes and even build mountains. The Indian subcontinent, for example, has been moving inexorably northward for millions of years, colliding with Asia like a slow-motion car wreck, the land at the edge of the collision buckling to form the Himalayas. Mount Everest and other mountains in the chain are still growing at a rate of about a half-inch per year.
Picking Up the Pieces
Geologists say it was a lurching collision between the Indian plate and the Burma plate, which grind together along a 750-mile long, north-south fault in the Indian Ocean, that triggered the recent earthquake off Sumatra, and the resulting tsunami.
The quake is believed to have shifted north Sumatra and smaller nearby islands by as much as 60 feet.
In human time, earthquakes that powerful are rare, but in the vastness of geologic time, they are commonplace.
"An earthquake of this magnitude, in this part of the world, has probably occurred about a million times since the breakup of Pangea,'' said Chris Scotese, a geophysicist at the University of Texas-Arlington. "No exaggeration.''
Geologists believe they understand, at least generally, what causes the plates to move and collide.
The Earth, they explain, is made up of pressurized layers. At the center is a hot metal core about 2,160 miles thick, the center of it solid and the outer layer molten. Then comes the hot, rocky mantle, about 1,800 miles thick. On top of that is the part we live on, a thin, cooler crust, perhaps 30 miles thick.
The crust is not solid and unbroken like the coating on a gumball. Rather, it is fractured into more than a dozen overlapping, rigid plates of rocky armor. The plates move relative to one another as they slide atop the hotter layers below.
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The overlapping points between plates are called subduction zones, and that is where the biggest earthquakes strike, changing Earth's map slightly each time. Volcanoes most often erupt in these boundaries between the plates.
Understanding these forces more precisely - in enough detail to predict earthquakes, for example - has proven elusive, however. For one thing, many of the boundaries between plates are covered by deep oceans, making them inaccessible to study. Even when plates come together on land, as they do in California, the real action occurs out of sight miles below the surface.
Only now, in central California, have scientists started to drill about two miles into a fault zone to learn more about these forces. But even that will offer only a blurry snapshot; the Dec. 26 earthquake occurred on a larger fault nearly six miles down.
Really big earthquakes - those like the Sumatra quake that noticeably rearrange the landscape - are so rare that there are few opportunities to study them. Until the Sumatra quake, it had been 40 years since a magnitude 9 temblor occurred - in Alaska. Like fishermen waiting for the big one, geologists can spend their entire careers waiting for the chance to study the huge temblor that never comes.
"A lot of what we see that is catastrophic occurs in the snap of a finger,'' says David Wald, a geophysicist for the U.S. Geological Survey. "And then nothing happens for hundreds of years.''
01-09-05 12:00 EST
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